1. Field of the Invention
This invention relates to a plugging liquid on emulsion basis, a method for its preparation and a method for plugging a zone of a subterranean formation penetrated by a drill hole.
2. Description of the Background of the Invention
A large number of gel forming compositions are known in the art for use as well control liquids in the drilling of wells for the recovery of oil and gas, for instance in cases where well liquids are lost to the formation or liquid or gas is flowing into the well from the formation. It is known to use polysaccharides for thickening purposes in such known gel forming compositions, see e.g. Norwegian Patents Nos. 116,667 and 161,625, U.S. Pat. No. 4,634,538 and British Patent No. 1,406,513.
It is also known to utilize as plugging liquids so-called "rheotropic" liquids. These liquids thicken when subjected to high shear stress. The rheotropic liquid must have physical characteristics enabling it to be easily pumped through the mud system (Condition 1) to the drill bit when leakages occur in the formation. The shear stress created when the liquid is forced through the nozzles of the drill bit, where the pressure drop will be in the range of about 50 to 120 bars, must result in a thickening of the liquid (Condition 2). The time before thickening must however be sufficient for the liquid to flow into and seal the formation.
U.S. Pat. No. 4,663,366 (Carr Corporation) discloses a rheotropic liquid of such a nature and an emulsion technique for controlling the thickening of the liquid. According to said patent, a polycarboxylic acid-containing water-in-oil emulsion is used wherein:
the oil phase contains dispersed therein a hydratable, water-swelling, hydrophilic clay such as bentonite, and additionally contains a solubilized surfactant consisting of a polyamine derivative, and PA1 the aqueous phase contains dissolved therein a polyacrylamide and a polycarboxylic acid. PA1 a) 5-50% by volume of a continuous phase containing: PA1 b) 50-95% by volume of a discontinuous phase containing: PA1 a) 20-40% by volume of a continuous phase containing: PA1 b) 60-80% by volume of a discontinuous phase containing: PA1 a) about 25% by volume of an oil phase containing: PA1 b) about 75% by volume of an aqueous phase containing: PA1 a) 5-50% by volume of a continuous phase containing: PA1 b) 50-95% by volume of a discontinuous phase containing: PA1 a) 20-40% by volume of a continuous phase containing: PA1 b) 60-80% by volume of a discontinuous phase containing: PA1 a) about 25% by volume of an oil phase containing: PA1 b) about 75% by volume of an aqueous phase containing: PA1 a) 5-50% by volume of a continuous phase containing: PA1 b) 50-95% by volume of a discontinuous phase containing:
It is critical that the bentonite be kept separated from the water until the required thickening of the liquid is initiated. For that purpose each droplet of the dispersed aqueous phase is coated with a membrane or a film of a polymeric material which is formed as a consequence of the aqueous phase being dispersed or emulsified in the oil phase of the emulsion. The film or membrane is formed as a result of the interfacial interaction between the polyamine derivative in the oil phase and the polyacrylamide and the polycarboxylic acid in the dispersed aqueous phase. When the emulsion is subjected to high shear forces, the protective film around the dispersed droplets in the emulsion is broken and the bentonite comes into contact with the water, thus enabling a swelling of the bentonite and consequently a thickening of the liquid. However, for the rheotropic liquid of U.S. Pat. No. 4,663,366 the obtainable ratio of the thickness of the liquid in Condition 2 to the thickness of the liquid in Condition 1 is lower than is desirable.
There is a need in well control operations for improved plugging liquids which (1) have the lowest possible viscosity in Condition 1, before they are subjected to high shear forces, (2) thickens to a high thickness in Condition 2 after having been subjected to high shear forces, (3) maintain their acquired thickness for a longer period of time, and (4) to the least possible extent undergo a thickening in Condition 1 as a result of the emulsion gradually deteriorating during storage or before it has had time to pass through the nozzles of the drill bit in the drill hole.